Gas analyzer system employing a gas chromatograph and a mass spectrometer with a gas switch therebetween



Oct. 7, 1969 P. M. LLEWELLYN ET AL 3,47

GAS ANALYZER SYSTEM EMPL NG A GAS CHROMATOGRAPH AN: A MASS SPBCTROMETERWIT A GAS SWITCH THEREBETWEEN Filed March 27, 1967 F|G.| l8 A TRAu F ERA DETECTORJR /l i RECORDER GAS EIII+CKI5 CHROMATOGRAPH 4 If f: I5 r---5? 1 4 mo oRR VENT DUMMY VENT 6 ""7"" CARRIER 1: w W I GAS SUPPLY 22 ,E2%? mmw m;

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CONSTITUENT H GAS FLOW H g INTENSITY 1.

TIME INVENTORS PETER M. LLEWELLYN n NEP. TTLEJOHN 3% TORNEY UnitedStates Patent 3,471,692 GAS ANALYZER SYSTEM EMPLOYING A GASCHROMATOGRAPH AND A MASS SPEC- TROMETER WITH A GAS SWITCH THERE- BETWEENPeter M. Llewellyn, Menlo Park, and Duane P. Littlejolm, Santa Clara,Calif., assignors to Varian Associates,

Palo Alto, Calif., a corporation of California Filed Mar. 27, 1967, Ser.No. 626,196 Int. Cl. B01d 59/44 US. Cl. 250-413 11 Claims ABSTRACT OFTHE DISCLOSURE A gas analysis system is disclosed which employs a gaschromatograph and a mass spectrometer for identifying the peaks of thegas chromatograph. A gas transfer switch is disposed between the gaschromatograph and the mass spectrometer for switching only portions ofthe time sequenced gas peaks of the gas chromatograph into the massspectrometer for identification by mass analysis. A'dummy carrier gasstream is provided which is normally switched into the input of the massspectrometer via a gas separator such that a normal carrier gasbackground is maintained in the mass spectrometer. The gas chromatographpeaks or portions of peaks which are switched into the mass spectrometerare carried in a carrier gas stream. The flow rate is controllable onthe dummy carrier gas steam to a fraction of the flow rate from the gaschromatograph for time spreading the gas chromatograph peaks as fed tothe mass spectrometer in order to obtain more accurate mass spectra. Ina preferred embodiment, the gas separator is of a dual membrane type forsubstantially removing the carrier gas from the sample constituentsbefore mass-analysis. Also the preferred embodiment includes a detectorand recorder in the vented gas chromatagraph gas stream downstream ofthegas switch for recording the gas peaks minus the sampled portionsthereof.

DESCRIPTION OE THE PRIOR ART Heretofore, gas analysis systems haveemployed a mass spectrometer for identifying the output peaks of "a gaschromatograph. Such systems employed a gas flow constriction and avacuum pump between the output of the gas chromatograph and the input ofthe mass spectrometer for reducing the gas pressure from atmosphericpressure at the outputof the gas chromatograph, to 1O Torr, at theinputto the mass spectrometer. A variable gas leak or vent into the input ofthe pump allowed control over the sample gas pressure applied to theinput of the mass spectrometer; In this way a crude control over gaspeaks passed to the spectrometer was obtained.

The problem with this prior art arrangement was that the crude controlover the sample pressure to the mass spectrometer was reflected aspressure changes on the output of the gas chromatograph column. Thesepressure changes produced fluctuations in the resolution of the gaspeaks obtained from the'gas chromatograph. In another prior art gasanalysis system, the output gas peaks of a gas chromatograph were fed toan infrared spectrometer at atmospheric'pressure and to a leak whichleaked gas to a mass spectrometer. A first gas switch was connected inthe output line between the gas chromatograph and the infraredspectrometer and mass spectrometer leak. Three additional gas switcheswere connected in the input system of gas lines to the gaschromatograph. In operation, the output peaks were sequentially andintermittently analyzed by closing the first gas switch and openingcertain ones and closing other ones of the three input gas switchesduring analysis of the first peak.

The gas switches stopped the flow through the gas chromatograph withoutadversely affecting resolution of the gas peaks. After analysis of thefirst peak, the gas switches were returned to their original position torestant gas flow through the gas chromatograph to produce subsequentoutput gas peaks which were analyzed by intermittently stopping andstarting the gas flow through the gas chromatograph in the manner aspreviously described for analysis of the first peak.

This arrangement of gas switches is relatively complex and to be avoidedif possible. This system for gas analysis is described in a paper titledInterrupted-elution Gas Chromatography Its Application, with EluateConcentration, to the Automatic Production of Simultaneous Infrared andMass Spectra," by R. P. Scott et al., paper 20, Sixth InternationalSymposium on Gas Chromatography and Associated Techniques, Institute ofPetroleum 1966, edited by A. B. Littlewood.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved gas analysis system employinga gas chromatograph and a spectrometer.

One feature of the present invention is the provision, in a system foranalysis of gases employing a gas chromatograph and a spectrometer, of agas transfer switch connected between the output of the gaschromatograph and the input of the spectrometer for switching onlyselected portions of the output of the gas chromatograph to thespectrometer without producing excessive pressure fluctuations in theoutput pressure of the gas chromatograph.

Another feature of the present invention is the same as the precedingfeature including a dummy carrier gas stream that is normally switchedby the transfer switch into the input of the spectrometer and whichprovides a carrier gas stream for the selected portions of the output ofthe gas chromatograph which are switched into the spectrometer.

Another feature of the present invention is the same as the immediatelypreceding feature wherein the dummy carrier gas stream is derived fromthe output of the second column of a dual column gas chromatograph.

Another feature of the present invention is the same as any one or moreof the preceding features including the provision of a gas separatorconnected between the gas transfer switch and the spectrometer forsubstantially eliminating the carrier gas and enriching the sampleportion of the gas stream reaching the spectrometer.

Another feature of the present invention is the provision of a gasdetector, such as a flame ionization detector, connected to the ventedoutput of the gas chromatograph downstream of the gas switch formonitoring the gas peaks of the gas chromatograph minus the selectedportions fed to the spectrometer.

Another feature of the present invention is the same as any one or moreof the preceding features including the provision of a constriction orvalve for controlling the flow rate of the dummy gas stream whichcarries the selected sample portion, such that by reducing the flow rateof the dummy stream to a fraction of the flow rate of the output of thegas chromatograph, the selected por- BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a schematic block diagram of a gas analysis system employingfeatures of the present invention,

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a gas analysis system employing features of the presentinvention. The gas analysis system includes a gas chromatograph 1 intowhich is introduced a sample to be analyzed. In the gas chromatograph 1the sample is mixed with a carrier gas stream and passes through a gaschromatograph column. The column causes the various gaseous constituentsof the sample to be separated in time within the carrier gas streamwhich emerges as the output of the gas chromatograph. Such constituentsmay be separated by as little as seconds and as much as 5 minutes andthe typical output contains numerous gas peaks of differing sizes in thecarrier gas stream. These gas peaks are detected by a gas detector suchas a flame ionization detector inside the gas chromatograph 1 and thedetected output is recorded as a function of time to produce achromatogram, as shown by line 2 in FIG. 2. The output gas stream of thegas chromatograph 1 is at atmospheric pressure. Typical carrier gasesinclude the permanent gases such as He, H N2, AI.

As seen from FIG. 2, the gas peaks of the chromatogram are separated intime and this is helpful in identifying the sample constituents.However, an extremely precise identification of the various constituentgases is obtained by feeding selected sample portions of the various gaspeaks to a mass spectrometer.

Thus, a mass spectrometer 3 is connected to the output of the gaschromatograph 1 via a gas separator 4 and a gas transfer switch 5. Thegas separator 4 is preferably of the dual stage membrane type as morefully described and claimed in copending U.S. patent application Ser.No. 555,613 filed June 6, 1966, now U.S. Patent 3,421,292 issued J an.14, 1969 as a continuation-in-part of U.S. patent application 511,576filed Dec. 6, 1965, now U.S. Patent 3,455,092, issued July 15, 1969 andboth assigned to the same assignee as the present invention.

Briefly, the gas separator 4 serves to eliminate the carrier gas fromthe gaseous constituents of the sample and, thus, to enrich the samplegases as fed to the mass spectrometer 3. The separator 4 comprises aconduit 6 with two membranes 7 and 8 as of 1 mil thick silicone rubberfilm formed on a porous substrate support plate, sealed across theconduit 6 in spaced relation. The sample constituents, such ashydrocarbons, are dissolved in the silicone rubber and diifuse throughthe membrane to the other side under an applied pressure differential.The permanent carrier gases are not appreciably dissolved in themembrane and, thus, are largely excluded. Differential flow rates of1:1000 through each membrane 7 and 8 are obtainable for certainhydrocarbons and carrier gases.

The mass spectrometer 3 typically operates at a pressure on the order of10" torr. The spectrometer 3 is evacuated to this pressure by a highvacuum pump, not shown, such as, for example, a liter per secondcoldcathode getterion multiple Penning cell pump. The sample and carriergas flow from the switch 5 passes into the first stage of the gasseparator 4 at atmospheric pressure. The gas stream is directed acrossthe surface of the first stage membrane 7 and out a vent to theatmosphere. A vacuum pump, not shown, which is operable at a pressure ofon the order of 10- torr evacuates the region between the two membranes7 and 8. A pressure of 10- torr exists on the spectrometer side of thesecond membrane 8. An enrichment of sample material on the order of 10to 10 is obtainable through the gas separator 4. By varying the pressurein the region between the gas separating membranes 7 and 8, between 80%and 2% of the sample material is passed through to the mass spectrometer3.

This pressured controlled throughput feature is described and claimed incopending U.S. application Ser. No. 626,- 194 filed Mar. 27, 1967, nowU.S. Patent 3,398,505, issued Aug. 27, 1968 and assigned to the sameassignee as the present invention. A

The gas transfer switch 5 comprises a shuttle valve member 11 axiallymovable between two positions in a housing 12 having a pair of inputports 13 and 14 and three output ports 15, 16 and 17. In a firstpositionof the 3 is vented to the atmosphere via a second flame ionizationdetector and recorder 18.

Thus, when the gas transfer switch 5 is in the first position, theoutput of the gas chromatograph is being vented to the atmosphere andbeing recorded in two places. A first recording is obtained internallyof the gas chromatograph 1. A second recording is obtained downstream ofthe switch 5 such that it will record the affects of the switch 5.

Also, when the switch 5 is in the first position, the dummy carrier gasstream is being fed to the mass spectrometer 3. The dummy carrier gasstream provides a continuously flowing carrier gas stream through theseparator 4 to the atmosphere. A throttle valve 22 is provided betweenthe dummy carrier gas supply 19 and the switch 5 for regulating orcontrolling the dummy carrier gas flow I rate. The throttle valve 22 maybe set to flow the dummy carrier gas stream at a selected fraction ofthe flow rate of the output gas stream of the gas chromatograph forreasons which will be fully described below. Alternatively the throttlevalve 22 may be set to flow the dummy stream at a higher flow rate thanthat of the chromatogram.

In the second position of the gas transfer switch 5, as indicated bydotted lines, the output gas stream from the gas chromatograph 1 isdirected via shuttle 11 to output port 16 which leads to the gasseparator 4 and mass spectrometer 3. Also, in the second position of theswitch 5, the dummy gas stream is directed via shuttle 11 to output port17 which is vented to the atmosphere.

The gas transfer switch 5 is solenoid operated via switch 24 connectedbetween a current source 25 and a solenoid 26 which operates the shuttle11. Typical operating times between the first and second positions ofthe switch 5 are on the order of 10 milliseconds.

In operation, a sample is introduced into the gas chromatograph .1.'Theswitch 5 is in the first position and the operator observes thechromatogram or internal detector output of the gas chromatograph. Whena peak appears in the output, which the operator wishes to identify bymeans of the mass spectrometer 3, he closes switch 24 and causes the gastransfer switch 5 to switch a selected portion of the gas peak to themass spectrom eter 3, as indicated by the cross hatched portion of FIG.2. The mass spectrometer 3 may have a gas handling capacity of 10-torr-liter/ sec. and would be flooded by large peaks if they were fed intheir entirety to the mass spectrometer 3. Thus, the gas transfer switch5 is operated in the second position only long enough to obtain asufficient amount of the peak for mass identification. For a large peak,this sampling period may only be a second or two, whereas, for a smallpeak, as indicated by the cross hatched portions under the curve of line2 the sampling peak is such an example. The composite peak is formed bytwo peaks as indicated by the dotted lines. The composite peak ischanging composition in periods of 2 to 10 seconds. If the entire peakwere fed to the mass spectrometer, the mass spectrometer would identifytwo separate gases but would not correlate each the separateconstituents with any particular portion of the composite peak. However,these portions of the composite peak can be separately identified bysampling the composite peak at points indicated by the cross hatching.

It turns out that for certain cycloidal type mass spectrometers 3 thatthey have optimum accuracy and linearity if the mass spectrum is run inabout 20 seconds. However, the typical small output peaks from the gaschromatograph 1 have a time span of less than 20 seconds. The selectedportion of the gas chromatograph output peak can be spread out, forexample, from 2 seconds to the desired 20 seconds by adjusting the dummycarrier gas flow rate to only that of the gas chromatograph flow rate.This is easily accomplished by proper adjustment of the throttle valve22. Thus, the operator actuates the gas transfer switch 5 for twoseconds or so to inject a two second portion of the sample peak into thestream leading to the mass spectrometer 3. This injected sample is thenpushed and carried along to the mass spectrometer 3 in the slower movingdummy carrier gas stream. This provides sample to the mass spectrometerfor the desired 20 seconds of the dummy stream has A the flow rate ofthe stream from the gas chromatograph.

Alternatively, relatively long and low intensity gas peaks can becompressed in time and, thus, increased in intensity by adjusting thethrottle valve 22 for a higher fiow rate than that of the output streamof the gas chromatograph. This latter mode of operation facilitatesidentification of certain relatively long and low intensity gas peaks.

One feature of the gas switch 5 is that it switches the sample stream toeither the vent or to the spectrometer 3 at atmospheric pressures suchthat the switching action does not produce pressure fiuctuations in theoutput of the gas chromatograph 1. Such pressure fluctuation, ifpresent, would cause unwanted line broadening and loss of resolution ofthe gas chromatograph 1.

Although the gas analysis system of FIG. 1 has been described asemploying a gas chromatograph and a dummy carrier gas stream, the dummycarrier gas stream may be replaced by the carrier gas stream from asecond gas chromatograph or the second column of a dual column gaschromatograph. This embodiment, which employs the second column of adual column gas chromatograph as the dummy carrier gas supply, has asecond output tubing 13' which is connected into the dummy gaspassageway 14 between the dummy carrier gas supply 19 and the throttlevalve 22. When using the output gas stream of the second column of adual column gas chromatograph 1 as the dummy carrier gas stream, thedummy carrier gas supply 19 can be eliminated or valved off.

Also more than one gas chromatograph may be employed in conjunction withthe dummy carrier gas stream. Such systems employing the gas transferswitch for switching the outputs of plural gas chromatographs to onemass spectrometer has the advantage of permitting one relativelyexpensive mass spectrometer to be used for identifying the outputs ofplural gas chromatographs. Also, the mass spectrometer may be replacedby other types of spectrometers such as an infrared spectrometer.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a gas analysis apparatus, means forming a gas chromatograph foranalyzing a sample and producing a plurality of sequential gas outputpeaks in a first stream of carrier gas, means forming a spectrometer foridentifying at least certain of the gas output peaks of saidchromatograph, the improvement comprising, means for producing a secondcarrier gas stream, means forming a gas transfer structure having a pairof separate input ports connected to receive said first and second gasstreams, respectively, said gas transfer structure having a pair ofoutput ports, means in said transfer structure for simultaneouslyopening a gas passageway interconnecting said first input port with saidfirst output port and for simultaneously opening a second gas passagewayinterconnecting said second input port with said second output port,means for switching said interconnecting means in said gas transferstructure for opening a third gas passageway interconnecting said firstinput port and said second output port for diverting said first gasstream to said second output port such that the flow of gas in saidfirst gas stream through said chromatograph is essentially uninterruptedby switching said interconnecting means in said gas transfer structure,and means forming a gas passageway connecting said spectrometer to saidsecond output port for sampling the gas fiow therefrom, whereby saidfirst gas stream and said second gas stream are sequentially switchableto said spectrometer without producing substantial pressure fluctuationin the first carrier gas stream flowing through said gas chromatograph.

2. The apparatus of claim 1 wherein said spectrometer means is a massspectrometer.

3. The apparatus of claim 1 wherein said gas chromatograph is a dualcolumn gas chromatograph, and said second carrier gas stream is derivedfrom the second column of said dual column gas chromatograph.

4. The apparatus of claim 1 including means for throttling the flow rateof the second carrier gas stream to less than the flow rate of the firstcarrier gas stream such that a selected portion of the gas peak outputof said gas chromatograph which is switchable into the second carriergas stream by said gas transfer means has its flow rate decreased as fedto said spectrometer means to facilitate spectral analysis thereof.

5. The apparatus of claim 1 including means for ad justing the flow rateof the second carrier gas stream to more than the flow rate of the firstcarrier gas stream such that a selected portion of the gas peak outputof said gas chromatograph which is switchable into the second gas streamby said gas transfer means has its flow rate increased as fed to saidspectrometer to facilitate spectral analysis thereof.

6. The apparatus of claim 1 including means forming a gas separatorconnected between said spectrometer means and said gas transfer meansfor substantially eliminating the carrier gas from and enriching thesample gas peak portion of the output gas stream of said gas transfermeans which is directed into said spectrometer means.

7. The apparatus of claim 6 wherein said gas separator means is amembrane separator.

8. The apparatus of claim 1 including means forming a gas detectorconnected to said first output port of said gas transfer structure formonitoring the non-diverted portions of the output gas stream of saidgas chromatograph.

9. The apparatus of claim 8 including, means for recording the output ofsaid gas detector means.

10. The apparatus of claim 1 wherein said gas transfer means has itsoutput ports, which are connected at various times through said gastransfer means to said gas chromatograph, operating at approximatelyatmospheric pressure.

11. In a method for gas analysis, passing a first gas stream containinga sample to be analyzed through a gas chromatograph to produce anefiluent first gas stream Containing sample constituents separated intime, producing a second gas stream of a carrier gas, passing theefiiuent first gas stream to a detector and recorder to detect thesample constituents and to record a gas chromatogram of the sample underanalysis, passing the second gas stream to a gas separator and thence toa mass spectrometer to maintain a certain background carrier gas signallevel in the output of the spectrometer, periodically diverting, withoutsubstantially interrupting the flow of the efliuent first gas streamfrom the detector and recorded'to the gas separator and massspectrometer for mass analying the detected constituents of the efiluentfirst gas stream, whereby the diverted portions of the efiiuent firststream are recorded in the gas chromatogram to be readily correlatedwith the detected peaks of the mass spectrometer.

References Cited UNITED STATES PATENTS 2,902,111 9/1959 Henke et a1.55--17 3,291,980 12/1966 Coates et a1.

RALPH G. NILSON, Primary Examiner S. C. SHEAR, Assistance Examiner

